Self-repairing materials taking their inspiration from living systems are known in which minor damage causes an automatic self-repair response, fracturing, cutting, etc. . . ).
An intelligent system based on similar principles would be very interesting for structures including an organic coating, whereby cracking or delamination of said coating could be at least partially repaired or wherein effects could be reduced without manual intervention. Such a coating with a self-repairing capacity could be considered to endow the protected structure with a substantially increased service life.
Two major self-repairing material concepts exist. The first consists of closing the crack, the second in filling it.
In the case of closing the crack, the two faces are drawn together to remove the stress concentration at the bottom of the crack: without such stress concentration, the crack cannot propagate. The two faces can be brought together by shape memory materials (metallic alloy or polymer).
When filling a crack, the crack is filled with a repair agent. The latter is a cross-linkable polymer, generally of the same nature as the coating, which will form a chemical bond between the two faces of the crack. It may also be a solvent, preferably for thermoplastics, which will cause the polymer chains either side of the crack to diffuse (in this case there is no cross linking/bonding between the two faces).
In the context of self-repairing materials using a repair agent, three points are crucial to implementation:                storing the repair agent;        transporting said agent to the crack; and        initiating the repair action.        
In the literature, repair agents are stored in hollow spheres or micro-capsules within the polymer and can repair the polymer in the event of a crack.
As an example, U.S. Pat. No. 6,518,330 describes a polymer comprising inclusions of micro-capsules or hollow spheres filled with a polymerizable agent and particles of a catalyst for the polymerization reaction.
In a further example which is similar to the preceding example, United States patent application US-A-2004/0007784 describes a polymer in which the catalyst particles are bonded to the surface of micro-capsules containing the polymerizable agent.
For such storage systems, the repair reaction is caused by the stress field in the crack. This causes the hollow spheres to break. The repair agent is then transported along the crack by capillary forces and polymerizes under the effect of the catalyst, thus plugging the crack.
Finally, in the case of such self-repairing materials it is necessary to have a repair agent in situ which is close to any possible cracks and ready to react during alteration of the material.
In the more particular case of structures comprising at least one metallic surface protected from external corrosion by cathodic protection, in accordance with the invention it is more particularly proposed to develop an assembly comprising an anti-corrosion coating for said surfaces in a corrosive medium (water, land) which is also protective and self-repairing. A typical use for said assembly, for example, concerns the protection of pipelines placed in seawater or buried, for the transport of effluents.
Effluent conduits and steel installations may be protected by the joint action of a coating and cathodic protection. The latter consists of placing the metallic surface under a sufficiently low electrical potential, ideally less than −0.8 V, and preferably in the range −0.8 V to −1.1 V/(Ag/AgCl), to minimize corrosion.
As an example, in the particular case of steel oil pipelines which are submerged in the sea, they are often protected from corrosion by the presence of a coating and by using cathodic protection. The latter consists of applying a potential that artificially places the metallic surface of the pipeline outside its corrosion potential, thus rendering it less corrodible than it was initially.                Cathodic protection may be achieved in two different manners:                    either by bringing the structure into contact with a second metal at a potential which is lower than that of the metal comprising the structure, which is then generally designated by the term “sacrificial anode”;            or by applying an impressed current which renders corrosion of the metal comprising the structure impossible.                        
According to the principle of cathodic protection, when cathodic protection is applied, the oxidation reaction (equation [1]) of the metal composing the wall (termed a metal in the present description) is discouraged and oxygen reduction (equation [2]) and/or proton reduction (equation [3]) is encouraged, or even water reduction at a very low potential (equation [4]). In practice, it is very difficult to control the applied potential, which sometimes leads to cathodic overprotection when the potential is too negative.oxidation of metal (corrosion): M=>M2++2e−  [1]reduction of oxygen: O2+2H2O+4e−=>4OH−  [2]proton reduction: 2H++e−=>H2  [3]water reduction: 2H2O+2e−=>2OH−+H2  [4]
If the coating has a defect such as micro-cracking caused, for example, during application of the coating, laying the structure, contact with the seabed or the land or by ageing of the coating, OH ions will become concentrated at the coating/metallic surface interface at the defect. The significant increase in pH in said zone has the effect of causing greater or lesser loss of adhesion of the coating, risking its detachment sooner or later.
On the one hand, the production of hydrogen associated with proton reduction (equation [3]) or, in the case of cathodic overprotection, the reduction of water (equation [4]) has the result of reducing the metal oxides present on the structure surface, this phenomenon causing a loss of adhesion of the coating on the metal and detachment thereof. In extreme cases, a high concentration of hydrogen may also cause breakage of the metallic structure.